Learn more about the Pigeonhole Principle and other powerful techniques for combinatorics problems
in our Intermediate Counting
& Probability textbook by USA Math Olympiad winner (and MIT PhD) David Patrick.
Pigeonhole Principle
Pigeonhole Principle
The basic pigeonhole principle says that if there are 
holes, and 
piegons (k>1), then one hole MUST contain two or more pigeons. The extended version of the pigeonhole principle states that for n holes, and $nk+j$ pigeons, j>1, some hole must contain k+1 pigeons. If you see a problem with the numbers n, and nk+1, think about pigeonhole.
Examples
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You could paste in these... (maybe, just a suggestion)
